alice
constraint
tutorial.

Constraint propagation is not a complete solution method.
It may happen that a space has a unique
solution and that constraint propagation does not find it.
It may also happen that a space has no
solution and that constraint propagation does not lead to
a failed propagator.

A straightforward example for the second case consists of
three propagators for

XYXZYZ

and a constraint store

This space has no solution. Nevertheless, none of the propagators is
inconsistent or can tell something to the constraint store.

To see an example for the case where a unique solution is not found by
constraint propagation, suppose we have interval propagators for the
constraints